The present invention relates to the mixing of a powder with a liquid, particularly for enabling a solid reagent to mix and react with a liquid reagent.
Various methods of mixing solids and liquids are known. Many involve the agitation of the components in a rotary mixer combined with their stirring by driven blades or other mixing equipment. Such equipment can be very expensive to operate and can require the components to remain in the equipment for a considerable length of time to ensure that mixing is thorough and any solid lumps that may be formed are broken down.
An object of the present invention is to provide a method for mixing a powdered solid with a liquid that is both efficient and can be put into practice relatively cheaply so that it is suitable for use even with relatively large quantities of the components.
Accordingly the invention provides a method of mixing a powder and a liquid comprising forming the liquid into a flowing film and dispersing the powder and directing it at the flowing liquid film so that it impinges thereon and mixes therewith. The film is preferably formed in adherence with a wall surface, the liquid preferably being supplied to an upper part of the surface so that it flows downwardly along the surface under gravity and the powder is dispersed and directed at the wall to mix with the liquid.
The invention further provides, in another aspect, equipment for mixing a powder with a liquid, comprising a mixing chamber, liquid inlet means in the upper part of the chamber for directing the liquid on to an upper part of a supporting surface in the chamber, the liquid inlet means and the supporting surface being so arranged that the liquid flows downwardly in adherence with the surface in use, and means for introducing the powder to the chamber at a position spaced from the liquid inlet flow, for dispersing the powder within the chamber and for directing it toward the said surface so that it impinges on the liquid flowing down it, in use, and mixes therewith.
The advantage of the mixing method of the invention is that the liquid, being formed into a film presents a large surface area to the powder while the latter, being dispersed before it hits the liquid, will be captured thereby with less tendency to form aggregates than when a powder and liquid are simply fed into a container and mixed. A uniform mixture can thus be obtained fairly readily without great expenditures of energy.
In preferred embodiments of the invention the film is formed in such a way that its surface area expands as it moves away from the point at which the film is formed; this may be achieved by suitable shaping, for example curving, of a surface on which it is formed. Moreover conditions are preferably such that the film is turbulent, which, together with its large surface area, enhances the capture of the particles and mixing thereof within the liquid flow.
Although it is conceivable that the mixing might be carried out in an open environment, it is preferably carried out in a closed chamber and, again, although the film might be formed at one side of the chamber with the powder being directed at that side, it is preferred to employ a circular-section chamber with the film formed so as to flow down substantially the entire peripheral wall with the powder being dispersed from the central region and directed at the entire circumference.
Rotary feed inlets could be arranged to supply both the liquid and solid into the chamber, the inlets rotating to cover the entire circumference and the powder possibly being blown into the chamber to disperse it. Much more preferably, however, the powder is fed through a central, axial inlet and is centrifuged to direct it at the circumferential wall while the liquid is supplied through an annular inlet or array of spaced inlets coaxially around the powder inlet. Centrifuge means provided in the chamber may disperse the powder as well as directing it at the wall but suitable gas jets may also assist the dispersal and/or direction of the powder.
Although the invention as defined above is applicable to the mixing of any powder and liquid, it has been developed with particular concern to the mixing of an hygroscopic powder with an aqueous liquid and, even more particularly, to the mixing of powder and liquid reagents which react together. In these circumstances it is important to keep the solid and liquid components separate until they are brought together in the flowing liquid film and, in particular, to keep the powder supply dry up to this point.
More particularly it is found, in practice, with the use of hygroscopic solids, that it is both difficult and important to keep surfaces adjacent the powder inlet to the mixing chamber dry as any moisture in this region can cause the powder to cake, the caking gradually building up with time. Eventually this provides a path for mixture to track back into the powder supply duct, causing caking within the duct which obstructs the supply itself and necessitates stoppage of the process for cleaning. At worst, in the case of hygroscopic materials which react exothermically with water, this can result in substantial and even dangerous overheating, particularly if the moisture reaches a supply container.
Accordingly, a further aspect of the invention comprises a method for mixing an hygroscopic powder with an aqueous liquid comprising supplying the liquid to a chamber so that it flows in adherence with a surface therein, supplying the powder to powder delivery and dispersal means which disperse the powder in the chamber and direct it at the said surface so that it impinges on the flowing liquid and mixes therewith while preventing liquid/solid contact on or adjacent the powder delivery means that could result in moisture tracking back to the powder supply, the supporting surface preferably comprising a wall of the chamber itself.
The chamber might be arranged with appropriate baffles to prevent liquid/powder contact near the powder inlet but it has been found preferable to provide a dynamic seal between the powder inlet and parts of the chamber that might be contaminated by the liquid, the seal being formed, for example, by a gaseous flow. In particular, in the preferred circular-section chamber described above, an annular gas inlet is provided coaxially around the powder inlet through which gas is supplied to sweep moisture away from this region. The gas may be air depending on the nature of the components to be mixed. This arrangement has the added advantage that the gas flow assists in the dispersal of the powder within the chamber.
In addition to the provision of the dynamic seal to prevent liquid/powder contact at the powder inlet, the above mixing method provides for the formation of the liquid into a film that adheres to a surface in the chamber, preferably the chamber wall. Indeed steps are preferably taken to ensure that the liquid does flow on the wall surface without any splashing within the container which could result in splashing back to the powder inlet. For this purpose the delivery of the liquid to the wall surface is controlled both in quantity and direction so as to avoid splashing. To this end, the liquid may be delivered to the surface in a direction substantially along the surface and concordant with the desired direction of flow.
In a particularly preferred embodiment, the chamber has a domed upper wall and the liquid is fed substantially tangentially onto it from an upper inlet. The domed shape, conveniently although not necessarily spherical, ensures the desired spreading of the film out from the inlet to increase the surface area available to the solid particles. The chamber wall may continue in a smooth curve from its maximum circumference to an axial outlet but a frusto-conical shape is preferred in the lower part of the chamber to speed the fluid flow and to enhance turbulence which improves the mixing of the solid and liquid components.
The conical taper also enhances vortical flow within the chamber which may be further promoted by the provision of swirler means within the lower part of the chamber or in an outlet duct therefrom. Such vortical flow not only creates turbulence in the liquid but also causes a depression in the chamber which enhances any gas inlet flow around the powder inlet and the dispersion and centrifuging of the powder. In particular, although this gas flow may be driven by auxiliary drive means with or without imparting rotation directly, it can be arranged for the gas to be drawn in solely by the depression caused by the vortical flow. Likewise, any swirler means provided may be driven to rotate but, in practice, it is found that a static radial diffuser device achieves excellent mixing.
With reference now again to the liquid inlet flow, it was indicated above that this should preferably be controlled both in direction and quantity to prevent splashing. The quantitative flow may be, to some extent, controlled by appropriate valving in a supply duct but it is found that splashing may occur particularly when the liquid supply to the mixing chamber is either started or stopped. The liquid flow is therefore preferably controlled immediately at the inlet to the chamber.
For this purpose, regulable valve means may be provided at the inlet to adjust the size of the inlet aperture and/or the direction of the liquid flow through this aperture. The valve means may be regulable from the exterior of the equipment by manual, electronic or other suitable controls but most preferably are operated automatically in response to a sensed flow of the liquid. Such sensing and control may, for example, be achieved electronically via appropriate sensors in the flow path but, for simplicity, the valve means are actuated by the liquid flow itself, the valve means being biased towards a condition in which a smaller liquid flow is directed, without splashing, onto an adjacent wall surface over which it is to flow, and being movable by the flow itself as this in creases, against the biasing force, to a position in which the larger flow is also directed onto the wall surface without splashing.
The valve means preferably include a valve member movable between a position in which the inlet aperture is of a minimum size to allow the smaller liquid flow and a position of maximum size to allow the larger flow. The inlet means are preferably arranged, as indicated above, to direct the inlet flow substantially along the adjacent wall surface rather than at a large angle to it, which would promote splashing, in all positions of the valve member.
In the preferred case of an annular inlet to a circular section chamber, the valve member is preferably also annular and mounted on a fixed part of the chamber surrounding the powder inlet. The valve member may be movable, or expandable, radially of the chamber to reduce the inlet size but, in a preferred embodiment, is slidable axially of the chamber between its positions of use under the action of resilient biasing means.
In preferred embodiments of the invention, the powder supplied to the mixing chamber is dispersed and directed at the peripheral wall at least partly by centrifuge means. Conveniently the powder is dropped from an upper inlet to the chamber onto the centrifuge means within the chamber preferably closely below the powder inlet and including blades, vanes or other members extending from a rotary shaft which can break up any lumps of powder as well as centrifuging it towards the peripheral wall. The rotary shaft preferably extends axially from the chamber to receive drive from a suitable motor. Most conveniently it extends upwardly from the chamber through the powder inlet.
The powder may in some circumstances be fed solely under gravity to the mixing chamber from a supply chamber above it but in most cases a metered supply is required in which case an auger or equivalent means may be provided between the supply chamber and the mixing chamber.
Metered supplies both of the solid and the liquid are particularly required when the invention is employed for mixing solid and liquid reagents so that they can react but, more generally, the invention further provides a method of reacting a solid reagent with a liquid reagent, including providing the solid in powdered form and mixing the powder with the liquid under reaction conditions by forming the liquid into a film that flows in adherence with a supporting surface and dispersing the powder and directing it at the flowing liquid film so that it impinges thereon and mixes therewith.
The equipment described above is particularly useful for carrying out this method as it enables the solid, in very finely divided form, to be brought into contact with the liquid film so that intimate contact between the two media is achieved very quickly and over a very large surface area which promotes very rapid reactions. Moreover, by virtue of the turbulent conditions that can be achieved in the liquid film, the liquid surface is continuously renewed so as further to promote capture of solid particles while reactions can continue within the body of the liquid as the solid particles are retained therein. Depending always on the nature of the reagents themselves, the flow rates of the two reagents can be adjusted to ensure that the reaction at least nears completion within the mixing chamber and within a very short stay time.
The equipment described above also lends itself readily to the carrying out of a reaction involving oxidation of one of the components by oxygen. Accordingly, the invention further provides a method of reacting a solid reagent with a liquid reagent and simultaneously oxidising a component that can be oxidised by oxygen, including providing the solid in powdered form, forming the liquid into a flowing film and dispersing the powder with the aid of a gas flow containing oxygen while simultaneously directing the powder at the flowing liquid film so that the powder impinges on and mixes with the liquid, the reagents being held under such conditions that the oxidisable component is oxidised by the oxygen in the gas flow and the solid and liquid reagents react in the flowing liquid film.
As indicated above, the liquid inlet of the chamber of the invention may conveniently be separated from the powder inlet by a dynamic air seal and this air may, in addition to helping to disperse the powder and preventing moisture from tracking back to the powder source, also supply the oxygen required for the oxidation reaction.
The equipment described above may include appropriate storage, delivery and metering means for delivering the two components to the mixing chamber in desired relative proportions. Also it may include a receiving vessel for the mixture, from the chamber where, if necessary, the reaction may be completed before the finished product is stored or discharged. Appropriate monitoring equipment for monitoring the finished product and/or the starting components may also be provided together with appropriate feed-back controls for changing conditions as necessary.
The method and equipment described above has been devised particularly with a view to treating acid mine discharges which often contain a variety of toxic metals, to enable liquid, specifically water, and solid products to be obtained which can be discharged safely into the environment or disposed of safely in appropriate dumps. To this end it is found possible to treat the acid discharge with a mixture of oxides to neutralise it or raise its pH to a slightly alkaline value in a continuous flow in the equipment described above, the reaction possibly being completed in ducts downstream of the mixing chamber, and to obtain a sedimentable suspension in which the metals are bound in non-toxic form. The invention further comprehends a method of treating mine discharges to render them safe and disposable as indicated above.
In a variant of the invention, the mixing method further comprehends a step of mixing additional liquid with the liquid/solid mixture formed in the flowing film or, considered alternatively, a two step mixing process comprising a first step in which a powder is mixed with a first quantity of liquid by forming the liquid into flowing film and dispersing the powder and directing it at the flowing liquid film so that it impinges thereon and mixes therewith and a second step in which a further quantity of liquid is mixed with the mixture resulting from the first step.
This variant method of the invention has the advantage that the relatively difficult process of mixing a solid quickly and thoroughly with a liquid is achieved by the first step of the method of the invention which can be carried out in the equipment described above, with appropriate devices being provided and measures being taken to ensure proper mixing even of hygroscopic solids; even large volumes of liquid can then be added easily without the need for expensive mixing equipment. In particular, although the initial solid/liquid mixture and additional liquid may be fed into a common vessel provided with power-driven agitators, such a solution would require an additional power input and it is found possible to achieve the required mixing by forcing the two flows to pass through a common duct provided with static mixer means which cause turbulent flow and hence mixing.
Various mixer blades or baffles may be envisaged but, in combination with the preferred equipment described above, a radial diffuser device similar to that used to enhance vortical flow in the primary liquid/solid mixture, may be used.
The liquid/solid and liquid flows may simply feed into a common duct provided with the mixer means but it is found that enhanced performance is achieved if the liquid/solid mixture is supplied to outlet means within a duct carrying the larger liquid flow, the flows being in the same direction, and the duct having a Venturi restriction substantially around the outlet means such that, in use, the depression caused by the flow through the Venturi constriction draws the liquid/solid mixture into the liquid flow. The outlet means may comprise an axial is outlet from a duct carrying the liquid/solid mixture and/or peripheral apertures in an end portion of the duct. In addition to enhancing the mixing of the solid/liquid and liquid flows, the Venturi effect may provide the required depression in the mixing chamber to cause air to be drawn into it.